1. Field of the Invention
This invention relates to electronic foot pedals for outputting electronic signals for controlling engine RPM and in particular it relates to an improved biasing arrangement for yieldably biasing a treadle toward a home position.
2. Background of the Invention
Modern engines for trucks utilize electronic foot pedals that output a signal proportional to treadle displacement. The signal is input to an engine controller which controls, as one of its functions, the speed of the engine.
The electronic foot pedals are self contained, requiring only an electrical connection between the pedal and the engine controller. The electronic foot pedal has a treadle that is pivotally mounted to a base with the treadle biased toward a home position by springs. To provide a safety factor and also to comply with DOT requirements, the electronic foot pedal has dual biasing springs to pivotally bias the treadle toward the home position. Two springs are provided in the event one of the springs fail. The springs are supplementary yet each spring has an adequate biasing force to pivot the treadle to its home position.
Many electronic foot pedals have a pivot arm or member disposed between the treadle and base to provide a multiplier effect for rotating a potentiometer more degrees than the treadle is pivoted. This provides a signal having a finer resolution.
The springs are often coiled wires, with one of the springs of a smaller diameter and nested inside of the other or springs of equal size mounted side by side. Each of the springs have a straight section extending from each end of its coil.
The springs are interposed between a treadle and a base or between the treadle and the pivot arm for example with the ends of the springs extending from one end of the coils exerting a force against the treadle and the ends extending from the opposite end of the coils exerting a force against the base (either directly or through the pivot arm). The force exerted by the springs against the base and the treadle thus provide a yieldable biasing force to urge the treadle to pivot away from the base.
It has been found that when the springs fail, it is almost always at the juncture where the straight section blends with the curved section of the coil. The straight sections extending from the coil of the springs are subject to a bending force rather than a tensile force. The force applied to the end of the extended straight section is near normal to the longitudinal axis of the spring. The straight section of the spring extending from the coil acts as a lever arm and the force applied to the end of the straight section thus tends to force a bending of the spring at the juncture of the straight section with the curved section of the coil. The spring experiences its greatest flexure in this area. This is one of the reasons that most of the breakage of springs occurs in this area.
The force applied to the coiled portion of the spring tends to wind the coil tighter. This force is distributed substantially over the mass of the effected portion of the spring and the larger the mass the less likelihood of failure. However the spring mass of a wire coiled spring has a practical limit imposed by the available space and in particular by the width of the pedal's treadle.
It is therefore an object of the present invention to reduce spring failure by providing a spring biasing means that is not subject to the torsional bending moment experienced by the coiled springs and also provides a greater mass of spring material within the available space.